This invention relates to the gasification of hydrocarbon-containing solids, particularly retorted shale.
It is known in the art to use destructive distillation, pyrolysis or retorting to extract hydrocarbons from hydrocarbon-containing solids such as shale, coal, tar sands, etc. In the case of coal or shale, the solids are first crushed and then heated by various means to an elevated temperature typically in the range of 700.degree. to 1400.degree. F or higher in a retorting vessel to drive off or liberate the hydrocarbons contained in the solid. During the mining of hydrocarbon-containing solids and particularly during the subsequent crushing of the solids for retorting, finely-divided solids are formed which frequently tend to end up in the product oil.
Because large quantities of shale must be processed to produce limited amounts of shale oil, most retorting processes are designed to operate on a continuous basis and thereby involve the movement of large quantities of solids. Processes for the continuous retorting of shale to produce oil have a common problem in that the product contains varying concentrations of mineral particulate matter. This particulate matter leaves the retorting vessel as a fine dust entrained in the product vapor or mist. In the condensed oil, this particulate matter leads to a difficult separation and quality control problems. Once the product plus entrained particulate matter contacts a surface whose temperature is below the boiling point of the product, a viscous, sticky, semi-solid mass tends to build up on the surface. The deposit has a significant (20-40%) organic content thus reducing yields. Furthermore, if this semi-solid deposit is allowed to accumulate it ultimately plugs the retort outlet orifice and downstream conduits and process equipment.
The problem of particulate matter in the product oil is further complicated in retorting processes wherein a stripping gas is passed over or through the hydrocarbon-containing solids. The added flow of the stripping gas tends to increase the quantity of entrained particulate matter and thus further increases the solids content problem in the condensed oil.
In some prior art processes, as shown for example in U.S. Pat. No. 3,784,462, one partial solution to the dust problem has been to enclose cyclone separators within the retorting vessel. In other processes, hot cyclones or electrostatic precipitators located outside of the gasification vessel are used to separate the particulate matter prior to condensation of the oil. However, the cyclones and electrostatic precipitators must be operated at very high temperatures and the product stream must be maintained at or above the highest temperature attained during the retorting process to prevent any condensation and accumulation of solids on processing equipment. However, maintaining the effluent stream at high temperatures is expensive from an energy standpoint and allows detrimental side reactions to continue, such as cracking, coking and polymerization, which tends to decrease the yield and quality of the desired normally liquid hydrocarbons.
The present invention involves the use of a rotating spiral-shaped conveyor in at least one gaseous discharge conduit of a gasification vessel, particularly a shale retorting vessel. The use of helical conveyors in conjunction with retorting vessels is old in the art. For example, helical and screw type conveyors have long been used to feed raw shale into retorting vessels as is shown, for example, in U.S. Pat. No. 2,664,389. Also, screw type conveyors have been used as the conveying means for transporting solids through retorting vessels as shown, for example, in U.S. Pat. No. 1,388,718. Similarly, helical shaped conveying devices have been used to remove retorted shale from retorting vessels as is shown, for example, in U.S. Pat. No. 1,475,901.